Classifications

• • G—PHYSICS
  • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
  • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
  • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
  • G03F7/70—Microphotolithographic exposure; Apparatus therefor
  • G03F7/70483—Information management; Active and passive control; Testing; Wafer monitoring, e.g. pattern monitoring
  • G03F7/70591—Testing optical components
  • G03F7/706—Aberration measurement
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J9/00—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength
  • G01J9/02—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength by interferometric methods
  • G01J9/0215—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength by interferometric methods by shearing interferometric methods
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B5/00—Optical elements other than lenses
  • G02B5/18—Diffraction gratings
  • G02B5/1838—Diffraction gratings for use with ultraviolet radiation or X-rays

Definitions

• the present invention is generally related to extreme ultraviolet (EUV) photolithography systems, and more particularly, to measuring wavefront parameters in a photolithographic system.
• EUV extreme ultraviolet
• Lithography is a process used to create features on the surface of substrates.
• Such substrates can include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits, and the like.
• a frequently used substrate for such applications is a semiconductor wafer.
• One skilled in the relevant art would recognize that the description herein would also apply to other types of substrates.
• a wafer which is disposed on a wafer stage (WS) is exposed to an image projected onto the surface of the wafer by an exposure system located within a lithography system.
• the exposure system includes a reticle (also called a mask) for projecting the image onto the wafer.
• the reticle is generally located between a semiconductor chip and a light source, usually mounted on a reticle stage (RS).
• a reticle stage RS
• the reticle is used as a photo mask for printing a circuit on a semiconductor chip, for example.
• Lithography light shines through the mask and then through a series of optical lenses that shrink the image. This small image is then projected onto the silicon or semiconductor wafer.
• the process is similar to how a camera bends light to form an image on film.
• the light plays an integral role in the lithographic process.
• the key to creating more powerful microprocessors is the size of the light's wavelength. The shorter the wavelength, the more transistors can be etched onto the silicon wafer. A silicon wafer with many transistors results in a more powerful, faster microprocessor.
• the problem of measuring the quality of the illumination beam is a persistent one for the lithographic applications.
• the particular environmental needs of an EUN Tool make disassembly a long and arduous task.
• the present invention is directed to a transmission shear grating and checkerboard configuration for EUV wavefront sensor that substantially obviates one or more of the problems and disadvantages of the related art.
• the present invention is also directed to improving the performance of a shearing interferometer used as an EUN wavefront sensor (WFS) so as to condition the illumination in the object plane as described and to make the WFS achromatic.
• WFS EUN wavefront sensor
• An embodiment of the present invention comprises a wavefront measurement system that includes a source of electromagnetic radiation.
• An imaging system focuses the electromagnetic radiation at an object plane, and directs the illumination uniformly.
• a first grating is positioned in the object plane for conditioning the illumination reaching the input numerical aperture pupil plane of the PO.
• the first grating includes a plurality of reflecting lines each formed by a plurality of reflecting dots.
• a projection optical system projects an image of the first grating onto the focal plane.
• a second grating is positioned at the focal plane.
• a detector is positioned in the fringe plane and receives the image of the first grating through the second grating.
• the first grating has a pitch that is Vz times (or, in other embodiments, 1 times) a magnification of the projection system times a pitch of the second grating.
• FIG. 1 shows a portion of an EUN photolithographic system.
• FIG. 2 shows an illustration of how a sensor module and a source module of the present invention fit within the photolithographic system.
• FIG. 3 A shows an example of a 2-D checkerboard grating.
• FIGs. 3B and 4 illustrate the use of an interferometer to produce shear wavefronts.
• FIG. 5 is another schematic illustrating the present invention as used within a photolithographic tool.
• FIG. 6 illustrates an example of interference fringes as they appear at the focal plane with the use of the present invention.
• FIGs. 7-11 illustrate examples of interference fringes seen at the focal plane with the use of the present invention.
• FIG. 12 illustrates one embodiment of a source module grating using a random pattern of reflecting dots.
• FIG. 13 illustrates a fringe pattern for a source module grating using 1/2 times a magnification of the projection system times a pitch of the second grating for achromatic operation.
• FIG. 1 illustrates a portion of an EUN photolithographic system 100, which includes an EUV source (not shown in FIG. 1).
• the system 100 also includes image optics (including mirrors M4 and M3), a pupil 101, a reticle 102 mounted on a reticle stage (RS, not shown) with an image of a pattern to be imaged onto a wafer 105, and projection optics (PO) 104 mirrors Ml and M6.
• the EUV radiation is then projected onto a wafer 105, which is mounted on a wafer stage (WS, not shown).
• WS wafer stage
• the reticle 102 is reflective in EUV systems, unlike photolithographic systems operating at longer wavelengths, such as deep ultraviolet, or visible, where the reticle 102 is usually transmissive.
• a sensor module 106 is placed on the wafer stage, and a source module 103 is placed on the reticle stage, to measure the wavefront.
• the sensor and source modules 106 may also be referred to as a wavefront sensor (WFS).
• WFS wavefront sensor
• FIG. 2 is another illustration of the wavefront measurement apparatus of the present invention, particularly as it can be incorporated into a photolithographic system.
• the source module 103 is placed on the reticle stage, and in one embodiment includes two orthogonally oriented gratings.
• the wavefront sensor (or sensor module 106) is placed on the wafer stage and includes a 2-D grating 201 and a CCD detector 202 that is positioned below the 2-D grating.
• the projection optics (PO) 104 remain the same as during normal exposure operation.
• the wavefront can be measured when imaging is not being performed.
• the reticle stage is moved, such that one of the gratings 203 in the source module 103 on the reticle stage is placed in the optical path, rather than the reticle 102 itself.
• the wafer stage is also moved such that the wavefront sensor is positioned to receive an image of the source module grating 203.
• the CCD detector 202 below the 2-D grating 201 then receives and measures the transmitted radiation.
• the reticle stage can then be moved to place a different diffraction grating in the optical path, so as to measure the wavefront with an orthogonal orientation of the source module grating 203.
• FIG. 3A illustrates one embodiment of the 2-D grating 201 of the present invention.
• a checkerboard grating can be used, with a carefully chosen pitch of the grating.
• Such a grating can be manufactured on a substrate of, e.g., 100 nanometers thick, made of a material that is transmissive at the exposure wavelength. For example, for 13.5 nanometers, examples of such transmissive materials include silicon and silicon nitride.
• the 2-D checkerboard diffraction grating 201 thus can be said to have a 50% duty cycle.
• the primary interference will be of zeroth order and the + and - 1 st order.
• Randomized nature, of diffuse scattering from source module 103 is expected to effectively wash out spatial variation in the wavefront across the projection optics 104 pupil.
• the pitch of the 2-D grating 201 is the length of the perpendicular square.
• the 2-D grating 201 also includes reflective (or opaque) regions, as may be seen in FIG. 3A. These reflective regions can be formed of materials that absorb EUV radiation (this case, for 13.5 nm exposure wavelength), such as nickel, chromium or other metals.
• the checkerboard grating pitch was chosen as 1.6 microns.
• the pitch has to be carefully chosen, to cause an appropriate angle for first order diffraction (as discussed below) for a particular shear ratio and numerical aperture.
• the shear ratio was chosen to be 1/30 th , although it will be appreciated by one of ordinary skill in the art that the invention is not limited to these particular numbers or dimensions.
• the output numerical aperture of the system is 0.25 (and the input numerical aperture is 0.0625, for 4X magnification), although the invention is not limited to this particular numerical aperture.
• the pitch of the 2-D grating 201 is chosen in one embodiment to provide a shear ratio of l/30 th , where the CCD detector 202 is in the fringe plane (i.e., below the focal plane of the system), and "sees" a pattern of fringes (an interferogram) or a number of overlapping circles, as will be discussed further below.
• the shear ratio is a measure of the overlap of two circles, where a shear ratio of zero represents perfect overlap. Note also that it is desirable for the CCD detector 202 to "see" only the zeroth order and the + and - 1 st order diffraction images, and to eliminate the + and - 2 nd order on diffraction images.
• the use of a checkerboard grating with square transmission and reflection areas, as shown in FIG. 3, is believed to be optimal.
• the first grating 103 is constructed to aid in eliminating unwanted orders. It is important, however, that whichever pattern of transmission and reflection areas is used, that it be a regular pattern that forms a 2-D grating. It will be appreciated however that other shapes, in addition to square shapes, are possible, e.g., circular reflective areas, or circular transmissive areas, etc., as long as the pattern is regular.
• the pitch of the linear grating of the source module 103 is preferably exactly 4X the pitch of the 2-D grating 201 of the sensor module 106.
• the pitch of the source module grating 203 is preferably 6.4 microns.
• the source module grating 203 can be manufactured accordingly to have a pitch 4X times the measured checkerboard grating pitch. This reduces the need for extreme precision in the manufacture of both sets of gratings simultaneously.
• Another embodiment of the 2-D grating 201 is a cross grating, such that two linear gratings of an appropriate pitch are essentially placed one on top of another, with each grating having the appropriate pitch dimension to result in an appropriate combined diagonal pitch. It is believed, however, that the checkerboard grating gives best results.
• a checkerboard grating or a cross grating can be used in the source module 103 instead of two separate linear gratings, although the use of a 2-D grating in the source module 103 complicates the detector readout and the analysis mathematics.
• the pitch of the source module grating 203 is also chosen to make interference between + and - 1 st order images disappear.
• FIGs. 3B and 4 illustrate the use of a pupil in a lateral shearing interferometer
• a wavefront 301 converges at a point in space, while emanating from a primary source.
• An image of a point source 302 exists at an entrance pupil 101.
• a partially transmitting film may be placed at the entrance pupil 101.
• a pinhole 303 is positioned at the entrance pupil 101. The pinhole 303 generates a transmitted wave 304 with a wavefront 311, which includes a diffracted spherical reference wave 305.
• the lateral shearing interferometer 310 creates one or more apparent sources, whose wavefronts 311 interfere to produce fringes 312.
• FIG. 5 is another illustration of the wavefront measurement system of the present invention, showing the source module 103 positioned in the object plane (reticle 102 plane, not labeled in the figures) and the projection optics 104.
• An image shearing grating 203 positioned on the wafer stage, and generates multiple wavefronts that are then detected in the sensor module 106.
• FIG. 6 illustrates the wavefront fringes (312 in FIG. 3) as seen by the CCD detector 202.
• sheared fringes for a single object space slit are shown, where the slit is positioned in front of an incoherent, diffuse source that fills the maximum numerical aperture and smoothes any wavefront ir-homogeneities.
• the bottom right-hand figure shows a fringe visibility function 601, with zeroth order and first order diffraction patterns.
• the 50% duty cycle on the grating 203 makes all even orders of the diffraction pattern invisible.
• the image space sharing grating 201 is shown, with a shear ratio of 0.5.
• FIGs. 7-11 illustrate exemplary wavefronts as seen by the CCD detector 202, for different shear ratios.
• the wavefront can be further analyzed by taking a number of images N at the CCD detector 202 while scanning the reticle stage by linewidth/N at a time.
• the images may then be combined to produce a better analysis than would be available from a single image.
• a particular problem that frequently exists in many EUV photolithographic systems is that the EUV source does not provide uniform information, but instead has a number of facets, or hot spots, in its exit pupil that result from use of flies' eye lenses in the optics of the EUV source. This results in a non-uniform wavefront at the input numerical aperture of the pupil of the PO 104, or sometimes, in underfilled numerical aperture of the PO.
• the particular system of one embodiment of the present invention has an input numerical aperture of 0.0625 for the projection optics 104, and an output numerical aperture of 0.25.
• FIG. 12 illustrates one proposed solution for overcoming these problems.
• each reflecting line of the grating is formed not of a continuous reflecting stripe (or line), but instead of a plurality of reflecting dots.
• the reflecting dots can be randomly scattered, as shown in FIG. 12, or can be arranged in a regular matrix pattern.
• the lines of the grating 1202 of the source module when seen “from far away,” appear to be solid lines, as discussed above.
• they when viewed "up close” (as illustrated at 1204), they are instead made up of a number of reflecting dots.
• the remainder of the material, for EUV application, would be absorbing.
• the diameter of the dots is between 70 and 120 nm, preferably close to 70 nm.
• the bottom portion of FIG. 12 shows the overall arrangement of two orthogonally oriented gratings that collectively form the source module grating 201 of the present invention.
• the reflecting dots can be arranged in two adjacent orthogonal patterns, each approximately 200 ⁇ m by 200 ⁇ m.
• the single diffraction pattern becomes a diffraction pattern within a diffraction pattern.
• each reflecting dot becomes a wavefront source, as viewed from the focal plane. Therefore, irregularities in intensity, particularly due to fly's eye facets of the source, will disappear, presenting a clean, regular image of the source at the focal plane.
• the reflecting dot pattern of the grating 201 also has the advantage that it overfills the 0.0625 numerical aperture of the projection optics, and utilizes as much light that is incident onto the grating 201 as possible. Furthermore, no additional pupil facets or pupil structure is introduced if illumination is spatially incoherent.
• the reflecting dot grating shown in FIG. 12 can be fabricated on a standard reticle blank. The dot diameter is preferably chosen to more than overfill the numerical aperture, so as to provide near-uniform pupil illumination.
• the reflecting dot grating 201 of the present invention is an example of a reflective EUV scattering device, that in this case has 0.4% efficiency for the large number of dots and first grating size, with an additional goal of maximizing the efficiency.
• Efficiency here is defined as the fraction of the illumination power falling within the desired numerical aperture after reflection from the device, as compared to a Lambertian diffuser.
• a tailored reflecting EUV diffractor fills the input numerical aperture of the projection optics, to eliminate pupil facets due to the Illumination Subsystem, and to maximize utilization of the illumination for optical system aberration measurements at EUV wavelengths using a lateral grating shearing interferometer. The last requires the equivalent of a grating placed in front of the incoherent extended source in the reticle plane. The first two require that the input illumination be reflected by a diffractor with a more favorable pattern than a Lambertian diffuser.
• the tailored reflecting diffractor may be an ensemble of micro-reflector diffraction-limited dots in the form of a grating.
• the individual reflecting dots in that ensemble would be placed at the "slits" of the 50% duty cycle "grating” and each individual reflecting dot of the size and shape to fill by diffraction the input numerical aperture of the optics being measured.
• the dot diameter might be 70 nm (to 210 nm).
• a single 70 nm dot would yield about 0.01 "detectable” EUV photon in EUV ILIAS.
• a 50% duty cycle "grating" 300 ⁇ m long, with 45 lines of pitch 6.4 ⁇ m, and with dots only along the central axis of each line could accommodate enough spots to yield up to 1,000 "detectable” photons which is more than adequate for EUV ILIAS.
• "Grating" lines filled with dots would give larger signals.
• a single (dotted) line would give an inadequate signal.
• a Lambertian diffuser and regular grating of similar form would give a barely adequate signal.
• the reflecting dots could have different heights so as to give them phase differences and so serve to eliminate the center bright spot of the prior random pattern.
• the random phases due to random heights serve the same purpose.
• the speckle in the presence of partially coherent illumination may be a problem for some applications.
• the reflecting dots can be placed regularly without the phase steps, and such regular placement may trade speckle for less troublesome very low frequency artifacts.
• the reflecting dots are placed randomly within the grating lines, speckle appears in the fringe pattern, as well as a bright spot at the center.
• the bright center can be eliminated by making the reflecting dots of random height with a standard deviation of many times the wavelength (i.e., OPD many times ⁇ plus a fraction.
• the overlapping fringe artifacts in the fringe plane can likewise be eliminated (but causing speckle) by making the dots of random height with an optical path difference standard deviation of many times ⁇ .
• the fringe artifacts may have less impact on fringe analysis.
• CA coherence area due to mask size and configuration
• An heuristic justification shows that the pitch of image space grating for a shear of 1/30 is 1.6 ⁇ m (i.e., 30x13.5/0.25).
• the grating is placed with same orientation over source whose pitch is magnification x 1.6 ⁇ m or 6.4 ⁇ m.
• the image of object space grating overlaps image space grating.
• object space grating slits place real sources at positions of image space grating apparent sources.
• FIG. 13 illustrates a fringe pattern for a source module grating using 1/2 times a magnification of the projection system times a pitch of the second grating for achromatic operation.
• an object space grating of less than 25% duty cycle and pitch equal to demagnification/2 x image space grating pitch will cause +1/-1 orders to interfere, 0/+1 and 0/-1 not to interfere, to improve achromaticity and maximize utilization of field source light.
• the first grating has a pitch that is Vi times a magnification of the projection system times a pitch of the second grating.
• a wide range of object space coherence function tailoring can be implemented in this manner. Note that the same effect can be achieved with image space spatial frequency filtering, although such an approach may be more difficult. Note also that this approach may be combined with the reflecting dots described above, or may be used by itself, without the reflecting dots.
• Object space grating is also important in obtaining fringe phase-shifting in two orthogonal directions (two object space gratings). Two separate gratings at 45 degrees are used to accomplish lateral motion. The cross-grating in image space is likewise at 45 degrees.
• Phase-shift one family of fringes can be accomplished by translating each object space grating. Translation causes the phase of the 1st order to vary with regard to 0th order and phase-shifts fringes.
• Achromaticity of a Grating Lateral Shearing Interferometer requires two conditions: the fringe spacing must be independent of source wavelength and the fringe pattern origin at each wavelength must independent of wavelength.
• GLSFs are often claimed to be achromatic, but that is true generally for only a few lower order aberrations and only for the fringe spacing part.
• the choice of orders for interference in GLSI's will affect the second part of achromaticity. That is, +1/-1 order interference has both fringe spacing and fringe origin achromaticity whereas 0/lorder interference has only fringe spacing achromaticity (for lower order aberrations). Expanding the fringe pattern of two adjacent orders in a Taylor series leads to:

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